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A service is a process or application which is running in the background, either doing some predefined task or waiting for some event. If you remember our process chapter, we learned about systemd for the first time there. It is the first process to run in our system; it then starts all the required processes and services. To know about how the system boots up,
read the bootup man page. Click here to read it online.

If you look at Unix/Linux history, you will find the first process which starts up, is also known as init process. This process used to start other processes by using the rc files from /etc/rc.d directory. In the modern Linux systems, systemd has replaced the init system.

Units are a standardized way for the systemd to manage various parts of a system. There are different kinds of units, .service is for system services, .path for path based ones. There is also .socket which are socket based systemd units. There are various other types, we can learn about those later.

Even if you start a service, you’ll find that after you reboot the computer, the service did not start at the time of boot up. To do so, you will have to enable the service, or to stop a service from starting at boot, you will have to disable the service.